Orthodromic and antidromic pacemaker circus movement tachycardia.

Pacemaker circus movement tachycardia (PCMT) during DDD pacing is usually sustained by retrograde natural and antegrade electronic atrioventricular (AV) conduction. As PCMT is often initiated by a ventricular premature beat (VPB) one method of its prevention is the programming of an atrial stimulus synchronously following a ventricular extrasystole. A patient is described with preserved antegrade, but without retrograde, i.e., VA, conduction. The optional pacemaker mode of synchronous atrial stimulation following a VPB caused an unusual PCMT sustained by retrograde electronic and antegrade natural AV conduction. This PCMT is similar to a natural reentry tachycardia, the most common variety of which (based on retrograde conduction) is termed antidromic and that which we describe is orthodromic.